1. Field of the Invention
Embodiments of the present invention relate to optimize the performance of organic Rankine cycle (ORC) power systems that utilize a working fluid and a geothermal heat source or similar heat sources.
More specifically, embodiments of the present invention relate to optimize the performance of organic Rankine cycle power systems that utilize a working fluid and a geothermal heat source or similar heat sources, where the cycles are binary power systems of an organic Rankine cycles utilizing a flow of a geothermal fluid or a similar heat source fluid. The present power cycles provide three improvements, all of which are intended to work together. Current, ORC systems have several limitations, which the present invention is intended to improve, overcome or solve.
2. Description of the Related Art
One primary limitation of an ORC system is the fact that the geofluid commonly used as a heat source fluid is usually highly mineralized, which limits the minimum temperature to which this geofluid can be cooled to provide heat for a geothermal power system. If it is cooled further than this limit, the mineralization will be deposited on the heat exchange apparatus, fouling it and interfering with the operation of the power system.
However, ORC power systems in current operation do not make maximum allowable utilization of their heat sources. Because the temperature of the cooling medium (air or water) varies over the course of each day, as well as by season and in response to the weather, the working fluid enters and exits the system's feed pump with different temperatures, depending on the coolant temperature. Thus, in real-world operation, an ORC system must operate so that, even on the coldest day (corresponding to the coldest possible temperature of the cooling medium), the exit temperature of the heat source always remains above the limit imposed by issues of mineralization.
Another factor that must be considered is that, the higher the boiler pressure, the higher the boiling temperature becomes. This means that as the pressure in the boiler is increased, the portion of heat available for the process of vaporization becomes smaller. That is, all of the heat available from a given heat source may be conceptually divided into two portions; the heat used for the vaporization of the working fluid and the heat used for the pre-heating of the working fluid from the temperature at the point just after the feed pump up to the boiling temperature of the working fluid.
But at the same time, the higher the boiler pressure, the more power output the system yields per given weight unit (pound, kilogram, etc.,) of working fluid. Thus higher pressure in the boiler increases output per weight unit of working fluid but decreases the total amount of working fluid that may be vaporized by the heat source. The goal in actual operation is to attain a point which corresponds to maximum power output, rather than just maximum heat source utilization. For each given allowable condensation temperature, there will be an optimal desired pressure in the boiler at which the system output will be maximized; this may or may not correspond to maximum possible heat source utilization. In certain parameters, higher pressure in the boiler will allow for increased output per unit of heat source fluid, even if the outlet temperature of the heat source fluid is not at the lowest allowable point.
Yet another limitation on the operation of ORC plants is that, because the working fluid is a hydrocarbon (e.g., isopentane), it must not be allowed to form a potentially explosive, fuel-air mixture at any point in the system. This is a potential issue in the system's condenser, and due to this fact, the pressure in the condenser must always be maintained at a pressure above atmospheric pressure; the usual operational standard is to maintain a pressure in the condenser of 15.693 psia or 1 psig, i.e., 1 psi above atmospheric pressure.
At sea level, this pressure corresponds to a temperature of condensation for the isopentane working fluid of 85.53° F. When the temperature of the cooling air becomes substantially lower than this temperature, the temperature of condensation must still remain constant. To that end, the flow of cooling air is reduced so that the temperature of condensation remains at 85.53° F. or higher, and thus the corresponding pressure remains at 1 psig. Thus any potential increase in power output that would otherwise be available from such a decrease in the temperature of the cooling media cannot be utilized. There is some economy attained from reducing the work of the fans that bring in the cooling air, but this is relatively small; the system's gross output is unchanged but the reduced fan energy costs do slightly improve net output. None the less, most of the potential for increased output available from the cooler air is not utilized.
The need to keep the outlet temperature of the heat source above the point at which mineralization begins to occur, the need to keep the pressure in the condenser above its mandatory minimum, and the desire to establish an optimal pressure in the boiler to obtain optimal power output for the system are all subject to a limitation inherent in a conventional ORC system.
In order to attain the optimum set of parameters of a power system, it is necessary to vary the boiling pressure and the flow rate of the working fluid, based on the coolant temperature, while carefully keeping both heat source outlet temperature and condenser pressure above their required minimums. However, the work done by the feed pump in a conventional ORC system is such that, for any given difference of pressure between the points before and after the feed pump, there is one specific and invariable flow rate of working fluid through the system.
At the same time, the flow rate that would be required for the optimal operation of the power system at a given pressure is not the same as the flow rate inevitably corresponding to that given pressure. Because a conventional ORC system has no capability to vary its flow rate independent of its pressure, a conventional ORC system operates in a suboptimal regime, i.e., chosen to maximize output as best as possible, while carefully staying within the limits imposed by the mineralization-based minimal allowable heat-source outlet temperature and minimal allowable condenser pressure.
One more additional issue with the conventional ORC technology is that, in order to regulate the flow rate through the turbine, i.e., a required capability, as the turbine has a maximum flow rate that it can accept and this must not be exceeded, an admission valve is installed prior to the turbine. The admission valve allows the flow rate through the turbine to be controlled, preventing the turbine from excess flow rate, but such use of an admission valve results in a pressure loss and corresponding loss of possible output.
Thus, there is a need in the art for systems and methods for implementing the systems that address these output limitations of conventional ORC power system.